AirPort: What is Network Address Translation?
Use of the Internet requires that your computer be identified with a unique address known as an Internet Protocol (IP) address. Unfortunately, there is a finite number of IP addresses available so Internet service providers (ISPs) typically assign only one IP address per subscriber.
Since many homes and small offices have multiple computers with access to the Internet, being limited to a single IP address can pose challenges. However, with NAT a small network of computers can share a single IP address to access the Internet.
The Apple AirPort Base Station provides NAT services to both AirPort-equipped clients and to clients connected to the AirPort Base Station via Ethernet.
To understand NAT, it is not necessary to understand exactly how IP addresses are structured. However, it is important to understand that IP addresses are not random numbers.
Consider your street address. You cannot simply choose your street address. It is a number defined in relation to a numbering scheme used for your street, the street name, and zip code. All of these are part of a larger organizational system which makes it possible to locate your house.
Similarly, an IP address contains information that conforms to a larger organizational system. This system enables data intended for your computer to find your computer making communication on the Internet possible.
With regards to NAT, it is best to conceive of your home network and the Internet as two distinct environments. As the Network Address Translator, an AirPort Base Station serves as a mediator between these two environments. What makes your home network environment different from the Internet is that your home network does not need to conform to the large, complicated organizational structure of the Internet.
A device providing NAT service needs only one IP address to send and receive data from the Internet (like you have only one street address). The computers on your local network are assigned IP addresses; however the address are not communicated outside of your local network.
So if every computer on the local network communicates with the Internet using the same address, how does the AirPort Base Station determine where to send the different incoming messages? How does it know that an email for Computer A is not for Computer B or C?
NAT uses an aspect of Internet addressing called ports. Normally incoming data is sent not only to an individual IP address, but to a particular Application in use on that computer. So, Web browser packets go to your Web browser port and email packets go to your email port, and so forth.
As far as the Internet is concerned, your network is a single computer allowing you to use one IP address. Through the use of ports within NAT, email messages or Web pages are delivered to the proper computer and application.
Note: Wireless Internet access requires an Internet service provider (fees may apply) and AirPort (or AirPort-compatible) wireless Ethernet card and base station. Some ISPs are not compatible with AirPort. For more information, see technical document 106590: "AirPort: Requirements for Wireless Internet Access"